Multistable auditory perception

While binocular rivalry has been studied since the 16th century, the study of multistable auditory perception is relatively new.[3] Diana Deutsch was the first to discover multistability in human auditory perception, in the form of auditory illusions involving periodically oscillating tones.[4]

Different experimental paradigms have since been used to study multistable perception in the auditory modality. One is auditory stream segregation, in which two different frequencies are presented in a temporal pattern. Listeners experience alternating percepts: one percept is of a single stream fluctuating between frequencies, and the alternative percept is of two separate streams repeating single frequencies each.

Other experimental findings demonstrate the verbal transformation effect. In this paradigm, the input is a speech form repeated rapidly and continuously. The alternating percepts here are words—for example, continuous repetition of the word “life” results in the bistability of “life” and “fly.” Prefrontal activation is implicated with such fluctuations in percept, and not with changes in the physical stimulus, and there is also a possible inverse relationship between left inferior frontal and cingulate activation involved in this percept alternation.[5]

The temporal dynamics observed in auditory stream segregation are similar to those of bistable visual perception, suggesting that the mechanisms mediating multistable perception, the alternating dominance and suppression of multiple competing interpretations of ambiguous sensory input, might be shared across modalities. Pressnitzer and Hupe analyzed results of an auditory streaming experiment and demonstrated that the perceptual experience that occurred exhibited all three properties of multistable perception found in the visual modality—exclusivity, randomness, and inevitability.[6]

Exclusivity was satisfied, as there was “spontaneous alternation between mutually exclusive percepts,” and very little time was spent in an “indeterminate” experience. Randomness also characterized the phenomenon, as the first phase of perception is longer in duration than subsequent phases, and then the “steady-state of the temporal dynamics of auditory streaming is purely stochastic with no long-term trend.” Lastly, the percept alternation was inevitable; even though volitional control did reduce suppression of the specified percept, it did not exclude perception of the alternative percept altogether. These similarities between perceptual bistability in the visual and auditory modalities raise the possibility of a common mechanism governing the phenomenon. In Pressnitzer and Hupe's subjects, the distributions of phase durations in the two modalities were not significantly different, and it has been speculated that the intraparietal sulcus, likely involved in crossmodal integration, could be responsible for bistability in both domains. However, the absence of subject-specific biases across the modalities contradicts the notion that a “single top-down selection mechanism were the sole determinant of the auditory and visual bistability.” This observation, along with evidence of neural correlates at different stages of processing, instead suggests that competition is distributed and “based on adaptation and mutual inhibition, at multiple neural processing stages.”[6]